1. Field of the Invention
The invention proceeds from a magneto-optic current sensor according to the preamble of Patent Claim 1.
2. Discussion of Background
With the preamble of patent claim 1, the invention refers to a prior art such as is known from a publication by K. B. Bohnert, H. Brandle and G. Frosio: FIELD TEST OF INTERFEROMETRIC OPTICAL FIBER HIGH-VOLTAGE AND CURRENT SENSORS, Tenth International Conference on OPTICAL FIBRE SENSORS, Glasgow, Scotland, Oct. 11-13, 1994, pages 16 to 19, published by SPIE--The International Society for Optical Engineering, Volume 2360. There, a fiber-optic current sensor is specified in which laser light passes via a fiber coupler and a fiber polarizer to a Y-coupler at which it is split in two linearly polarized component waves. The first wave runs via a phase modulator, an optical fiber approximately 90 m long, which maintains the linear polarization, to a first .lambda./4 fiber loop. The linear polarization is tranverses in the loop into a circular polarization. The circularly polarized light traverses a fiber-optic sensor coil with a diameter of 42 cm and 20 turns, which has a relatively low birefringence. The laser light emerging from the sensor coil, still circularly polarized in the ideal case, is transformed back again into linearly polarized light in a second .lambda./4 fiber loop, and runs back to the Y-coupler via a further polarization-maintaining glass fiber approximately 90 m long and the phase modulator. The second component wave traverses the optical circuit with the same polarization states in the opposite direction. The two returning waves are brought to interfere in the Y-coupler. The resulting interference signal passes to a photodiode via the fiber polarizer and the fiber coupler. The magnetic field of the electric current, which is surrounded by the fiber coil, generates an optical phase shift between the two oppositely directed light waves in the coil. The phase shift is detected as a corresponding change in the interference signal. The bending-induced, linear birefringence of the .lambda./4 loops, and thus the relative phase lag, are a function of temperature. The linear birefringence of the fiber coil likewise varies with temperature. These effects are strongly marked particularly in the case of low temperatures, because in this case the plastic protective cladding of the fiber is usually hardened and generates additional birefringence. The measuring sensitivity of the sensor changes as a result. These effects often exhibit a hysteresis-like behavior, and so this signal can scarcely be corrected exactly even when the temperature is known.
In a relatively limited temperature range between 0.degree. C. and 70.degree., the relative phase lag varies in the case described by approximately 7.degree. in the sensor coil and by 4.degree. in the .lambda./4 loop. The relative measuring error was .+-.0.15% for a current of 900 A and a constant temperature.
DE-AS 2445369 discloses a magneto-optic measuring transducer for high-voltage current measurements, in which the optical fiber used as current sensor, made from glass and having an inside diameter of 57 .mu.m, has a liquid core of hexachlorobuta-1,3-diene. The aim thereby is to eliminate the strongly temperature-dependent stress birefringence in the case of optical conductor coils made from graded-index fibers.
DE 4304762 A1 discloses a sensor head for a fiber-optic current-measuring device using a polarimetric detection method, without .lambda./4 time-delay elements, in which a twisted low birefringent LB fiber guided around a current conductor and made from silica glass is arranged in the interior of a capillary made from quartz and having a diameter in the range of 0.2 mm-0.5 mm and is held at the end virtually without force at fused splice points. The torsional stress on the LB fiber caused by the twisting is transmitted onto the capillary via the splice points and via bonded joints containing silicone. A plastic protective cladding normally surrounding the sensor fiber can, however, harden at low temperatures and cause disturbing birefringence. The light introduced into the sensor fiber is not guided in an oppositely directed fashion, with the result that interference effects from the sensor fiber do not compensate each other.
Reference may further be made to the publication by G. Frosio and R. Dandliker, Reciprocal reflection interferometer for a fiber-optic Faraday current sensor in: Applied Optics, Vol. 33, No. 25, Sep. 1, 1994, pages 6111 to 6122 for the relevant prior art. There, the sensor coil is mirrored at the end face. In this case, the same temperature dependencies occur as for the current sensor in the conference report named at the beginning.